WO3 work function enhancement induced by filamentous films deposited by resistive heating evaporation technique

Tungsten trioxide (WO3) films are of great interest due to their electronic properties which can be tuned by surface nanostructuring leading to enhanced efficiency for gas sensing, energy storage or electrochromic applications. Resistive heating of tungsten (W) filaments at pressures of few Pa in an oxygen O2 atmosphere has already demonstrated its capability to form porous, micro/nano-structured, cheap and fast films making it suitable for industrial applications. This work presents stoichiometric WO3 films produced by applying a current into a W filament for pressures in the range of 2 to 20 Pa. The increase of the pressure above 7.5 Pa led to amorphous WO3 films with a newly filamentous morphology. As a function of the pressure, the work function (WF) of the films was determined in-vacuo by aims of Ultraviolet Photoelectron Spectroscopy (UPS). In addition to the high surface to volume ratio of the films, the WF exhibited an increase from 5.8 eV, for a conventional WO3 film, to a maximum of 8.7 eV at 20 Pa. This change corresponds to an increase of the WF of about 50 % compared to 5.8 eV making our films suitable for a large variety of applications. The highering of the WF is mainly induced by the change of morphology to filamentous structures, the defect induced by the amorphous structure and the nanometric size of the filaments leading to the confinement of the electrons in the valence band for our WO3 films. Additionally, a change from p to n-type polarity is reported in-vacuo when increasing the pressure from 7.5 to 20 Pa with the formation of an intermediate band at around 0.8 eV below the Fermi level. Additionally, molybdenum trioxide (MoO3) and vanadium pentoxide (V2O5) filamentous films were deposited via the resistive evaporation technique in an O2 atmosphere. Similarly, copper, aluminium and molybdenum wires were used, leading to the formation of islands having foam-like structures analogous to the WO3.